1. Field of the Invention
The present invention relates generally to semiconductor processes and, more particularly, to a method for fabricating a deep trench in a substrate.
2. Description of the Prior Art
Along with the development of miniaturization of various electronic products, the design of the dynamic random access memory (DRAM) elements has to meet the requirements of high integration and high density. DRAMs with trench capacitor structures have become one of the main structures of high-integrated DRAM products used in the industry. This kind of DRAMs contains trench capacitors fabricated inside deep trenches that are formed in a semiconductor substrate by an etching process so as to effectively reduce a size of the memory cell and utilize space of chips well.
Typically, to form a high-aspect-ratio deep trench in a silicon substrate, a thick silicon oxide hard mask such as an undoped silicon glass (USG) layer or boron-doped silicon glass (BSG) layer is first deposited on the silicon substrate. Thereafter, a lithographic process is carried out to transfer a deep trench pattern onto the silicon oxide hard mask. After the silicon oxide hard mask is patterned, a dry etching process is then performed to etch the silicon substrate that is not covered by the patterned silicon oxide hard mask, thereby forming the deep trench.
However, the conventional method for forming the deep trench suffers from several drawbacks. The difference of an etching selectivity of the silicon oxide hard mask and of the silicon substrate is not high enough, which leads to the profile control problem during the formation of the deep trench in the silicon substrate. The profile control problem becomes worse when the dimension of the deep trench becomes smaller and the trench depth becomes deeper. Therefore, there is a strong need in this industry to provide a method for forming a deep trench with very high aspect ratio, wherein the sidewall profile of the deep trench is precisely controlled.